Recent requirements have led to controlling of operations of safety devices in accordance with a weight of an occupant seated on a seat for a vehicle, for the purpose of enhancing performances of safety devices, such as a seat belt, an air bag, and so on.
WO2006/011597 discloses therein an occupant weight measurement device. The occupant weight measurement device includes: a left and right pair of fixed lower rails fixed to a vehicle; a pair of movable upper rails provided so as to be movable in backward and forward directions on the respective fixed lower rails; a load sensor mounted on each movable upper rail and having a rod extending vertically upwardly; and a rectangular frame mounted above the pair of movable upper rails and formed with installation holes. The rods of the load sensors are vertically upwardly inserted into the installation holes of the rectangular frame, respectively and, thus the rectangular frame is mounted on the load sensors.
As described above, because the rectangular frame, which is shaped in advance, is mounted on the load sensors with the rods of the load sensors vertically upwardly inserted into the installation holes of the rectangular frame, occurrences of initial strain and initial load are restrained. As a result, it is possible to measure a weight of an occupant more precisely than an occupant weight measurement device, by which components for the rectangular frame are mounted on the load sensors and then assembled to the rectangular frame.
In the aforementioned occupant weight measurement device, load, which corresponds to a weight of an occupant, is converted into an electrical signal by a strain gauge at a sensing-portion of the load sensor. The electrical signal however, is influenced by even slight variations in electrical resistance. Therefore, in general, an amplifier having an electronic circuit is arranged in parallel, so that the variations in electrical resistance are amplified to be read. Meanwhile, various electromagnetic waves are induced by various in-vehicle equipments, such as drivers, lighting equipment, and so on, and influence on complex electronic circuits, so that an erroneous measurement and improper operations may occur. In light of the foregoing, in general the amplifier is housed in an amplifier case made of metal such as aluminum, and the amplifier and the amplifier case are both connected to earth, thereby shielding electromagnetic waves (noise). However, the aluminum-made amplifier case costs highly so that recent requirements have led to a resin-made amplifier case. Although the resin-made amplifier case is appropriate to reduce weight and to save a mounting space, it may not excel at reducing noise possibly contained in an electrical signal.
A need exists for an occupant load sensor for a seat of a vehicle, which is not susceptible to the drawback mentioned above.